The present invention relates to electric motors, and, more particularly, to multiphase DC brushless motors (typically three-phase motors) and to a start-up procedure for accelerating the rotor of such a motor until it reaches a predetermined speed.
A brushless motor has a permanent magnet rotor and a stator including a certain number of windings (commonly three) typically connected in a star or polygonal (i.e., triangle or delta) configuration. In addition to the conventional star or polygonal configurations, the windings may also be configured according to a so-called xe2x80x9cindependent phasexe2x80x9d configuration, where both terminals of each phase winding may be accessed externally and driven independently from the other phase windings.
In the most common case of star or triangle configured three-phase motors, the windings are driven by integrated circuits whose output stage generally includes a full wave three-phase bridge circuit made up of six bipolar or metal oxide semiconductor (MOS) power transistors. A typical output stage and three-phase DC brushless motor for connection thereto is shown in FIG. 1.
The most typical driving mode associated with a motor of this type is the so-called xe2x80x9cbipolarxe2x80x9d mode, in which at each instant two windings are driven while the third is in a high impedance state (tristate). The phase windings being driven are switched according to a cyclic sequence that should be synchronized with the instantaneous position of the rotor (i.e., of its magnetic axis or axes). In a bipolar driving mode, this may be easily established by monitoring the sensed back electromotive force (BEMF) of the phase winding that is in the high impedance state, or through dedicated position sensors.
To improve the performance of the system, the windings should be switched such that the motor may operate at the maximum efficiency level. This is achieved by maintaining a precise phase relationship between the current forced through the winding and the BEMF induced thereon. Start-up is accomplished by switching to supply the windings according to a certain switching sequence. This is done to induce a rotation to a successive position in the desired direction at a progressively increasing speed until the motor reaches a certain speed.
It is not possible to verify that the motor is indeed rotating in the desired manner by sensing the BEMF during such an acceleration phase, as it is done during the normal running of the motor. Indeed, until an adequate speed is reached, the back electromotive signal does not have a sufficient amplitude to be reliably used to sense the speed and position of the rotor.
A method for controlling the speed of the rotor includes sensing the position of the rotor at predetermined intervals of time while the motor is excited according to an electric current path chosen as a function of the position. This is done to induce the desired rotation. In particular, U.S. Pat. No. 4,876,491 to Squires et al. discloses a method in which the speed of the rotor is controlled by detecting the position of the rotor at predetermined periods of time. This is done by carrying out measurements on all the windings and, consequently, selecting the appropriate excitation current path. Basically, the disclosed method includes excitation phases, during which an acceleration is impressed upon the rotor, alternated with rotor position sensing phases during which the speed remains practically constant.
To rapidly bring the motor to a desired speed, it is desired to increase the duration of the excitation phases as much as possible and minimize the duration of the sensing phases. According to the above method, the excitation phases have an equal duration of time. Thus, at high speeds there exists the possibility that, by exciting a winding for a relatively long period, the rotor may surpass the excited winding and be slowed down. On the other hand, if the excitation phases were too short, the acceleration characteristics of the rotor would be reduced. In addition, each sensing phase would be relatively long, and therefore a longer time will be required to bring the rotor to the required speed.
It is an object of the present invention to provide a start-up procedure for a brushless motor including excitation phases whose duration are progressively reduced as the speed of the rotor increases and vice-versa, and sensing phases whose duration are diminished with respect to prior art methods.
This and other objects, features, and advantages according to the invention are provided by a method including determining the start-up position of the rotor, forcing a current through the motor""s windings for an established period of time and, at the end of each of the excitation phases, determining the position of the rotor. A distinguishing feature of the invention is that the duration of the excitation phases is not constant as in prior art methods. On the contrary, it is updated depending upon the number of consecutive times in which the rotor is sensed to be in the same position or in a different position.
The position of the rotor may be sensed by inductance measurements carried out at the end of each excitation phase. The inductance measurements may include forcing short current pulses into the windings and determining which of them has the minimum electric time constant (i.e., inductive sensing). Further, such a test need not be performed on all the windings but rather on a restricted number of them, i.e., on those functionally closest to the previously sensed position of the rotor.
The start-up procedure of the invention may be terminated to proceed with a classic driving mode of the motor based on sensing BEMF when the speed of the rotor has reached or surpassed a predetermined value, or when the rotor has simultaneously reached the desired speed and position. Additionally, the procedure of invention can be stopped and an alarm signal generated if the desired speed is not reached after a certain maximum allowable time has elapsed from the beginning of the start-up.